Argument of periapsis

Fig. 1: Diagram of orbital elements, including the argument of periapsis (ω).

In astronomy, the argument of periapsis (ω) is a way of talking about the orbit of a planet, asteroid or comet. It is also known as the argument of perihelion or the argument of perifocus. It is the angle (starting from the center of the orbit) between an orbiting body's periapsis and its ascending node. Periapsis is the point when the orbiting object comes the closest to the thing it is orbiting around; for example, the moon is at periapsis when it is closest to the Earth. The ascending node is one of two places where an orbiting object passes through the reference plane, an imaginary flat surface which runs through the object being orbited around. The size of the angle depends on which way the object is orbiting.

The angle is measured in the orbital plane and in the direction of motion. For specific types of orbits, words such as "perihelion" (for Sun-centered orbits), "perigee" (for Earth-centered orbits), "pericenter" (general), etc. may replace the word "periapsis".

An argument of periapsis of 0° means that the orbiting body will be at its closest approach to the central body at the same moment that it crosses the plane of reference from south to north. An argument of periapsis of 90° means that the orbiting body will reach periapsis at its northmost distance from the plane of reference.